1. Field of the Invention
The present invention relates to an exhaust gas purification system for an internal combustion engine which is particularly effective to reduce the emission of nitrogen oxides (NO.sub.x) of the engine exhaust gases.
2. Description of the Prior Art
Environmental pollution caused by gases exhausted from engines and, particularly, from those of automobiles has been a serious problem. The main harmful components of the engine exhaust gases are nitrogen oxides (NO.sub.x), carbon monoxide (CO), hydrocarbon (HC). Exhaust gas recirculation system (which is so-called "E.G.R. system") and secondary air supply system have been used to reduce the harmful components of the engine exhaust gases.
The prior art E.G.R. systems, which have been used to reduce the emission of NO.sub.x, are classified into a plate type and a manifold type. In the first type, an E.G.R. passage extends from an exhaust pipe of an engine to an intake pipe upstream of a throttle valve of a carburetor and a fixed throttle member is provided in the E.G.R. passage. In the second type, the E.G.R. passage extends from the engine exhaust pipe to the intake pipe downstream of the throttle valve and an adjustable valve is disposed in the E.G.R. passage for the control of the flow of the exhaust gases back into the intake pipe.
In the first or plate type E.G.R. system, because exhaust gases flow back into the intake pipe upstream of the throttle valve, the engine back pressure, which is a function of the amount of engine intake air, is a function of the amount of E.G.R. Thus, the amount of E.G.R. is advantageously proportional to the amount of engine intake air. The E.G.R. system of this type, however, has disadvantageous problems that a deposit of foreign material is formed on the throttle valve, an advance port is blocked, the carburetor and related components of the engine suffer from thermal influence, parts made of aluminum alloys are corroded and icing occurs at a low temperature.
In the E.G.R. system of the second or manifold type, the above disadvantageous problems hardly occur because the engine exhaust gases are recirculated directly into the engine intake manifold downstream of the throttle valve. In this type of E.G.R. system, the E.G.R. flow is a function of the intake manifold vacuum and thus is increased and decreased in light and heavy load operating conditions of the engine, respectively. Thus, valve means are required for controlling the E.G.R. flow such that it is not influenced by the engine intake manifold vacuum. In a conventional E.G.R. system, the control valve means are controlled by means of intake vacuum so that the valve is closed to interrupt the E.G.R. flow only at the engine operating condition to which the E.G.R. flow is disadvantageous to the engine operation, the intake vacuum providing on-off signals depending upon the positions of the throttle valve in the intake pipe. In another conventional E.G.R. system, either intake vacuum or venturi vacuum is used to actuate E.G.R. control valve so that the valve opening is decreased and increased in light and heavy load operating conditions of the engine, respectively, in order to prevent E.G.R. flow from being influenced by intake vacuum. In any case, however, the influence of intake vacuum on the E.G.R. flow could not completely be eliminated to such an extent that the E.G.R. flow was proportional to the intake air. Thus, in the prior art E.G.R. system, E.G.R. flow was unduely increased relative to the intake air flow at a light load operating condition of engine to cause surging and misfires at this operating condition, whereas the E.G.R. flow was decreased relative to the intake air flow at a heavy load engine operating condition with resultant decrease in the intended effect of E.G.R. system to minimize emission of NO.sub.x. With the prior art E.G.R. system, therefore, it has been difficult to reduce emission of NO.sub.x without adverse affect on the engine operation.
In the case where a secondary air supply system comprising an air pump has been employed to feed a secondary air into an exhaust manifold of engine, all the air discharged from the pump has normally been supplied to the exhaust manifold. The secondary intake air has influenced the engine back pressure to vary the difference between the engine back pressure and engine intake vacuum with resultant increase in the above-discussed difficulty.
In the past, the secondary air has also been fed into the exhaust port of the engine cylinder provided with exhaust gas delivery port of E.G.R. system. Thus, the secondary air has also been recirculated back into the intake pipe of the engine, so that the air-fuel ratio of the air-fuel mixture introduced into engine cylinders has been made larger than a predetermined air-fuel ratio for the engine. Thus, in the case where the predetermined air-fuel ratio for an engine was relatively small (i.e., the engine was designed to be operable with relatively rich air-fuel mixture), the introduction of the secondary air into the intake pipe increased the air-fuel ratio beyond the predetermined one and toward the stoichiometric air-fuel ratio with resultant increase in the emission of NO.sub.x. On the other hand, in the case where the predetermined air-fuel ratio for an engine was relatively large (i.e., the engine was designed to be operable with a relatively lean air-fuel mixture), the introduction of the secondary air into the intake pipe of the engine unduely increased the air-fuel ratio (i.e., the air-fuel mixture was unduely diluted), which adversely affected the engine operation.